STEM CELL TRIALS
therapy, doctors learned that a high threshold number of total cells was important for ensuring treatment success.
Although doctors are still unable to determine the specific dosages of the rarer HSCs, for these treatments, total cell count is an adequate surrogate for HSC dosage. Not so for HSCs in umbilical cord blood units that are used to treat children. As many as 18% of cord blood units fail children because their unknown HSC dosage is inadequate. In these early days of stem-gene clinical trial practice, similar detrimental effects are certainly at play. So far, the answer to the title question has not been provided. Here it is. Until very recently, no method was available to quantify the dosage of tissue stem cells. Several technologies are commonly misrepresented as able to determine stem cell dosage. However, experts in tissue stem cell medicine know that they can’t. The first is flow cytometry, which is perhaps an understandable error. Flow is very sensitive, but relies on specific biomarkers to detect and quantify sub-types of cells. However, there are no biomarkers that specifically identify tissue stem cells. The biomarkers commonly used in flow, like CD34 and CD90, also detect more abundant non-stem committed progenitor cells, which do not have the long-term tissue cell- renewing ability of tissue stem cells.
16 | H1 Virtual Events: Review and Summary Handbook
The colony-forming unit (CFU) assay is widely used to evaluate and certify cord blood units for clinical use. Like flow, it also detects committed progenitor cells, but cannot distinguish them from HSCs. The one previous assay able to distinguish stem cells from committed progenitor cells is the SCID mouse-repopulating cell (SRC) assay. Some stem-gene companies use it, but it is an assay looking for a better solution. A single dosage determination requires as many as 50 mice and 16 weeks to get a result that is highly unreliable; and it only works for HSCs.
Q. Why should stem-gene clinical trial sponsors count their stem cells? A. The simple answer to this question is if they do, they can reduce their costs, design and evaluate their clinical trials better, and improve the effectiveness of their treatments. A second equally important answer is that now they can. A new method called kinetic stem cell (KSC) counting was recently reported for conveniently counting any human tissue stem cells, including HSCs and MSCs. New attention to knowing the stem cell dosage of treatments is also developing in regulatory agencies like the FDA. So, the present moment may be ideal timing for supply vendors to begin considering tissue stem cell counting as an important stem-gene clinical trial supply need.
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